Viscosity is the single most important lubricant performance characteristic.
The emphasis is not solely on its bulk property but extends to how viscosity interacts with temperature and pressure, significantly affecting a lubricant’s efficiency and functionality in various conditions. This multifaceted impact makes understanding viscosity crucial for anyone involved in selecting or using lubricants.
While viscosity is often simplified to the concept of “thickness,” such a definition, although rudimentary, sets the stage for deeper understanding. Viscosity essentially measures a fluid’s resistance to flow. For instance, water, being “thin,” flows readily, while honey, much “thicker,” requires additional force to pour. This fundamental observation relates directly to a lubricant’s performance in machinery, where viscosity determines the effort needed to move or maintain the flow of the fluid under different conditions.
Viscosity’s scientific definition encompasses its resistance to deformation under shear stress. Imagine two large plates with a fluid between them; as one plate moves, it drags the fluid along due to viscosity. The force required to maintain this movement against the fluid’s resistance, balanced by internal friction, translates into a measure of viscosity. This relation is quantitatively expressed as viscosity being proportional to the shear stress divided by the velocity gradient, revealing the intricate relationship between fluid dynamics and lubricant performance.
The distinction between dynamic (absolute) and kinematic viscosity further refines our understanding. While both measure a fluid’s resistance to flow under force, kinematic viscosity incorporates the fluid’s density into the calculation and is the form of viscosity that is mostly shown on used oil analysis reports, as well as defining the ISO viscosity grades. It offers a nuanced view relevant in specialized applications where fluid density varies significantly. This differentiation is vital in accurately assessing lubricant performance across diverse industrial and mechanical contexts.
Viscosity measurement can be approached through various methodologies, each suited to particular aspects of a fluid’s behaviour under specific conditions. Methods include spinning a disc in a lubricant to gauge resistance (absolute viscosity) or observing the time a fluid takes to flow between two points under gravity (kinematic viscosity). These techniques underscore the multifaceted nature of viscosity and its critical role in lubricant analysis, selection, and application.
Lubricants are home to a plethora of viscosity grading systems, such as ISO, AGMA, and SAE, each tailored to specific application domains, from industrial machinery to automotive engines. Understanding these grades and their interrelations is key to selecting the right lubricant, ensuring it meets the operational demands of the equipment while optimizing performance and longevity.
ISO Viscosity Grades for Lubricating Oils
While they appear intimidating, the ISO sequence is a repeating set of numbers. 10, 15, 22, 32, 46, 68. These can be multiplied by 10 or 100 to obtain the remaining grades. Each grade has a tolerance of ±10%.
SAE Viscosity Grades
SAE viscosity grades for engine oils are standardized classifications established by the Society of Automotive Engineers (SAE) to indicate the oil’s viscosity characteristics. These grades help users select the appropriate oil for their engines based on temperature conditions and performance requirements.
The SAE viscosity grades are divided into two main categories: winter grades (denoted by a “W”) and high-temperature grades. Winter grades, such as SAE 0W, 5W, 10W, 15W, 20W, and 25W, measure the oil’s viscosity at low temperatures to ensure proper engine starting and lubrication in cold conditions. The lower the number before the “W,” the better the oil performs in colder temperatures.
High-temperature grades, such as SAE 20, 30, 40, 50, and 60, indicate the oil’s viscosity at 100°C (212°F), which corresponds to the engine’s operating temperature. Higher numbers represent thicker oils that maintain better lubrication under high temperatures and heavy loads.
Multi-grade oils, such as SAE 5W-30 or 10W-40, combine the properties of both winter and high-temperature grades, offering a broader range of protection across various temperature conditions. These oils are designed to provide adequate flow in cold weather while maintaining sufficient thickness to protect the engine at operating temperatures.
Gear oils are measured on a different scale. The general rule of thumb is, if the SAE grade is above 60, the oil is an automotive gear oil.
HTHS (High-Temperature High-Shear) viscosity is a measure of an engine oil’s viscosity under conditions of high temperature and high shear rate, which are typical in engine bearings and other critical components. This parameter is crucial for ensuring adequate lubrication and protection in modern engines, especially under severe operating conditions.
HTHS viscosity is measured at 150°C (302°F) using specialized equipment that simulates the shear forces encountered in an engine. The value is expressed in millipascal-seconds (mPa·s).
The main significance of HTHS viscosity is its ability to indicate the oil’s performance in maintaining a protective film between moving parts under high stress and temperature. Oils with higher HTHS viscosity values provide better protection against wear and reduce friction, which is essential for engine longevity and efficiency. However, higher HTHS viscosity can also result in increased fuel consumption.
SAE classifications for engine oils also specify minimum HTHS viscosity requirements to ensure that oils meet the necessary standards for protecting engines under demanding conditions. For instance, oils labelled as SAE 0W-20, 5W-20, and 10W-30 must meet certain HTHS viscosity thresholds to be considered suitable for use in specific engine applications.